A conventional telephone conversation apparatus with a hands-free function has a construction having a microphone mounted above a cone speaker, which outputs speech sounds of a telephone conversation party received by a mobile phone.
On the other hand, speech sounds of a user acquired by the microphone are transmitted via the mobile phone to a mobile phone of the telephone conversation party.
However, since the diaphragm of the cone speaker has a cone-like shape rather than a flat shape, frequency characteristics of the speech sounds output from the front of the speaker differ from frequency characteristics of the speech sounds output from the rear of the speaker.
In addition, the distance from the front of the speaker to the microphone differs from the distance from the rear of the speaker to the microphone.
Accordingly, even in the condition where the microphone is mounted above the cone speaker, the speech sounds output from the front of the speaker are not canceled by the speech sounds output from the rear of the speaker at the input to the microphone.
Thus, the speech sounds of the telephone conversation party received by the mobile phone are sent back to the mobile phone of the telephone conversation party, thereby arousing an echo.
To prevent the echo from being produced, the conventional telephone conversation apparatus includes an inverting circuit (echo canceling circuit) for inverting the audio signal captured by the microphone, and combines the audio signal inverted by the inverting circuit with the audio signal output from the speaker (see Patent Document 1, for example).
Patent Document 1: Japanese patent application laid-open No. 2-224498/1990 (from page 9 to page 10, and FIG. 1).
With the foregoing configuration, the conventional telephone conversation apparatus can cancel out the audio signal output from the speaker by the audio signal inverted by the inverting circuit, thereby being able to prevent the echo from being produced. However, the audio signal output from the inverting circuit has a phase delay corresponding to the inverting processing duration by the inverting circuit from the phase of the audio signal output from the speaker. Thus, the phase characteristics or frequency characteristics of the two audio signals cannot be matched completely, which offers a problem of making it difficult to completely suppress the echo.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide an acoustic apparatus and telephone conversation apparatus capable of preventing the production of the echo.